Sometimes when I work late in the library, I hear a faint shuffling sound coming from the stacks. Is there someone lurking, did someone fall asleep in a study carrel? I do a quick walk-through and check again. Nope. No one's there.
I shrug to myself, turn off the lights and go home.
This would explain why I can never find anything in the 500s.
(thanks to everyone who sent this!)